The answer is buffer overflow. Input drops occur when the ingress interface’s receive buffer is full and cannot accept more packets, typically because the ingress rate exceeds the interface’s ability to process or forward traffic to the internal switch fabric. On the JNCIA-Junos exam, this concept tests your understanding of queue management and packet loss causes—distinguishing buffer exhaustion from physical-layer issues like cable faults or speed mismatches, which would instead produce errors or link flaps. A common trap is assuming input drops stem from duplex mismatches, but those cause CRC errors, not buffer overflows. Remember: input drops mean the buffer is overwhelmed, not the cable. For a quick memory tip, think “drops = buffer stops.”
JNCIA-JUNOS Operational Monitoring and Maintenance Practice Question
This JNCIA-JUNOS practice question tests your understanding of operational monitoring and maintenance. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Refer to the exhibit. An engineer notices that input drops are increasing. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Buffer overflow
Input drops occur when the ingress interface's receive buffer is full and cannot accept more packets, typically due to the ingress rate exceeding the interface's ability to process or forward packets to the internal switch fabric. In Junos, this is often caused by microbursts or sustained oversubscription, not by a speed mismatch (which would show as errors or link flaps) or cable faults (which cause physical-layer errors). Buffer overflow is the correct answer because input drops directly indicate that the packet buffer has been exhausted.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Speed mismatch
Why it's wrong here
Would typically cause CRC or framing errors.
✓
Buffer overflow
Why this is correct
Input drops indicate buffer overflow or congestion.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Cable fault
Why it's wrong here
Would likely cause CRC or framing errors.
✗
CRC errors
Why it's wrong here
CRC errors are zero in the exhibit.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse input drops with CRC errors or assume a speed mismatch is the cause, but Junos explicitly separates input drops (buffer overflow) from physical-layer errors, and the question's exhibit would show no CRC errors or link flaps, isolating the issue to buffer exhaustion.
Detailed technical explanation
How to think about this question
Input drops are tracked in Junos under the 'Input drops' counter in 'show interfaces' output, and they increment when the receive ring (a fixed-size FIFO buffer) overflows. This can happen even with no errors on the wire if traffic arrives in bursts that exceed the interface's ability to DMA packets into memory. A common real-world scenario is a 1 Gbps interface receiving a microburst from a 10 Gbps uplink, causing transient buffer exhaustion without any physical-layer faults.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the JNCIA-JUNOS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Operational Monitoring and Maintenance — This question tests Operational Monitoring and Maintenance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Buffer overflow — Input drops occur when the ingress interface's receive buffer is full and cannot accept more packets, typically due to the ingress rate exceeding the interface's ability to process or forward packets to the internal switch fabric. In Junos, this is often caused by microbursts or sustained oversubscription, not by a speed mismatch (which would show as errors or link flaps) or cable faults (which cause physical-layer errors). Buffer overflow is the correct answer because input drops directly indicate that the packet buffer has been exhausted.
What should I do if I get this JNCIA-JUNOS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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